Thermal circuit protector

ABSTRACT

A thermal circuit protector for automatically opening a circuit when the ambient temperature becomes too high. The protector comprises a fusible pellet disposed inside a conductive case. A first conductor electrically contacts the case while a second conductor is electrically insulated from the case. Contact is made from the first conductor to the second by three electrically conductive balls which contact the case and the second conductor. A first spring maintains the electrical contact between the case, the balls and the second conductor. A second spring acts to break the contact between the case, the balls and the second conductor when the fusible pellet melts.

United States Patent [191 Henry [451 Dec. 25, 1973 1 THERMAL CIRCUITPROTECTOR [75] Inventor: Wesley D. Henry, Logansport, Ind.

[73] Assignee: Essex International, Inc., Ft. Wayne,

Ind.

[22] Filed: Feb. 20, 1973 [21] Appl. No.: 333,947

3,519,972 7/1970 Merrill 337/407 Primary ExaminerHarold BroomeAlt0rney-Robert D. Sommer [57] ABSTRACT A thermal circuit protector forautomatically opening a circuit when the ambient temperature becomes toohigh. The protector comprises a fusible pellet disposed inside aconductive case. A first conductor electrically contacts the case whilea second conductor is electrically insulated from the case. Contact ismade from the first conductor to the second by three electricallyconductive balls which contact the case and the second conductor. Afirst spring maintains the electrical contact between the case, theballs and the second conductor. A second spring acts to break thecontact between the case, the balls and the second conductor when thefusible pellet melts.

3 Claims, 4 Drawing Figures Q -PAIENTEUI1EB25 ms SHEET 1 [IF 3PATENTEUHEE2 197s 3.781. 737

SHEET BUY 3 FIG.3

PAIENTEI) HER??? I975 SHEET 3!)? 3 1 THERMAL CIRCUIT PROTECTOR Thisinvention relates generally to circuit breaking devices and pertainsmore particularly to a thermal fuse which interrupts or opens anelectric circuit when a predetermined ambient temperature is reached.

Many instances exist when it is desirable to protect electrical devicesfrom excessive heat. Electric motors can be damaged if they overheat.Thus it is desirable to protect them from overheating. As a furtherillustration, thermal protectors are useful in shutting off flammableliquid and gas sources when a fire has occurred.

Accordingly, it is an object of this invention to provide a thermalprotector for electrical circuits which has a precise operatingtemperature and which automatically interrupts the current flow in anelectric circuit.

This invention relates to a thermal circuit protector having acylindrical, tubular, electrically and thermally conductive case with anintegral closure at one end and an electrically nonconductive closure atthe other end. A first conductor makes electrical contact to the casewhile a second conductor extends through the insulating end closure. Afusible pellet which melts at a prescribed temperature is held in placeby a spring assembly. The spring assembly also maintains threeelectrically conductive balls in conductive engagement with the case andthe second conductor while the pellet is solid. When the ambienttemperature reaches the proper level, the fusible pellet melts and thespring assembly acts to move the three conductive balls out ofengagement with the second conductor to break the circuit. I

Other features and advantages of the invention will be apparent fromthis description, the accompanying drawings, and the appended claims inwhich:

FIG. 1 is a longitudinal cross section ofa thermal circuit protectorconstructed in accordance with the present invention;

FIG. 2 is a cross section along line 2-2 of FIG. 1;

FIG. 3 is a longitudinal cross sectional view of a second embodiment ofthe invention; and

FIG. 4 is a longitudinal cross sectional view of a third embodiment ofthe invention.

In the FIG. 1 a thermal protector, generally indicated by referencenumeral 10, comprises a cylindrical, tubular, electrically and thermallyconductive case 11 having closures 12 and 13 at each end. The closure orplug 13 is held in the case 11 by an inwardly extending flange 17 whichis formed after the device has been assembled. A sealing compound 29 mayseal the closure 13.

A first conductor 18 electrically contacts the case 11 at closure 12. Asecond conductor 19 extends through the bore 16 of plug 13 and isinsulated from the case 11 by the plug 13. A collar 20 prevents secondconductor 19 from being extracted from the thermal protector 10. The endof second conductor 19 is beveled as shown at reference numeral 21.

A temperature sensitive fusible pellet 22 may be of any suitable typenow well known in the art and which 1 may be purchased on the market byspecifying the desired collapsing temperature. These pellets have arigid body which can withstand any pressures exerted on them by thethermal protector assembly while the ambient-temperature is below thecollapsing tempterature.

When the ambient temperature exceeds the collapsing temperature,however, the pellet melts.

A plate 23 having a slightly smaller surface area than that of thepellet 22 acts to distribute the force exerted by pressure spring 24over the area of pellet 22. The outer diameter of the plate 23 issmaller than the inner diameter of the case 11 providing a pathway 25for the molten fusible material when the pellet 22 melts.

Pressure spring 24 forces three balls 26 into conductive engagement withcase 11 and the beveled end 21 of conductor 19. The balls 26 and the endof conductor 19 may be made of copper or other conductive material andmay be coated with silver to increase conductivity. A second spring orrelease spring 27 exerts a force on conductive balls 26 opposing that ofpressure spring 24. The pressure spring 24 and release spring 27 arechosen so that when the pellet 22 is solid and uncollapsed, pressurespring .24 exerts a greater force on balls 26 than release spring 27 tomaintain contact between case 11, balls 26, and the beveled end 21 ofconductor 19. The release spring 27 sits on plug extension 14 and isprovided with an end turn 28 whihch is smaller than the other turns.

When the ambient temperature reaches the collapsing temperature of thepellet 22, the pellet will melt and molten material will flow throughthe areas 25. Pressure spring 24 will therefore be allowed to expand,exerting less pressure on balls 26. When pressure spring 24 has expandedsufficiently, the force exerted on balls 26 by release spring 27 will begreater than that exerted by pressure spring 24. Thus, release spring 27will move the balls 26 out of engagement with conductor 19 opening anycircuit connected to conductors l8 and 19.

The embodiment shown in FIG. 3 is similar in operation to that shown inFIG. 1 and the same reference numerals are used to indicate the samefeatures in both FIGS. 1 and 3. In FIG. 3 plate 30 has been added and islocated between pressure spring 24 and conductive balls 26. Plate 30 isidentical to plate 23 and acts to distribute the force of pressurespring 24 exerted on balls 26.

The embodiment shown in FIG. 3 operates in the same manner as theembodiment shown in FIG. 1. Thus, when the pellet 22 collapses thepressure spring 24 will be allowed to expand. After pressure spring 24has expanded sufficiently, release spring 27 will exert a greater forceon balls 26 than does pressure spring 24 moving balls 26 out ofengagement with conductor 19.

In the embodiment shown in FIG. 4 electrical contact is made to thethermal circuit protector by conductor 18 which electrically contactsthe case 11 at integral closure 12. Insulating plug 13 having plugextensions 14.and 15 and bore 16 occupies the end of case 11. Aninwardly extending flange 17 holds plug 13 within the case '11. The endclosure may be sealed by electrically insulating sealing compound 29.

Fusible pellet 22 occupies the inside of case 11. Plate 23 acts todistribute the force exerted by pressure spring 24 evenly over pellet22. Pathway 25 provides a path for molten fusible material.

Electrical contact is made from conductor 19 to the case 11 by threeelectrically conductive balls 26. Plate 31 has a raised portion 32 whichprevents the balls 32 from rolling to the center.

Finally, release spring33 exerts a force on an insulator 34. Insulator34 has a bore 35 through which conductor 19 extends. Balls 26 rest in anindented area 36 of insulator 34.

In operation the embodiment shown in H6. 4 is similar to the previoustwo embodiments. However, the release spring 33 does not directly exerta force on balls 26. lnstead release spring 33 exerts a pressure onballs 26 indirectly through insulator 34.

Numerous modifications and changes can be made from the disclosedembodiments without departing from the true spirit of the invention asdefined in the claims.

What is claimed is:

1. A thermal circuit protector which comprises:

a tubular, electrically and thermally conductive case;

a normally solidfusible pellet occupying one end of said case;

an insulating plug occupying the other end of said case having a boreextending therethrough;

a first conductor electrically contacting said case;

a second conductor extending through the bore of said insulating plug;

electrically conductive balls normally in electrical contact with theinside of said case and said second conductor; and spring means formaintaining said balls in electrical contact with said case and saidsecond conductor when said pellet is solid and for breaking saidelectrical contact when said pellet melts. 2. The thermal circuitprotector as claimed in claim 1, wherein said spring means comprises:

a pressure spring exerting a first force on said conductive balls; and arelease spring exerting a second force on said conductive balls, saidsecond force opposing said first force, said second force being smallerthan said first force when said pellet is solid, said second force beinglarger than said first force after said pellet melts to disengage saidconductive balls and said second conductor. 3. The thermal circuitprotector as claimed in claim 2, wherein an insulating member isdisposed between said conductive balls and said release spring.

1. A thermal circuit protector which comprises: a tubular, electricallyand thermally conductive case; a normally solid fusible pellet occupyingone end of said case; an insulating plug occupying the other end of saidcase having a bore extending therethrough; a first conductorelectrically contacting said case; a second conductor extending throughthe bore of said insulating plug; electrically conductive balls normallyin electrical contact with the inside of said case and said secondconductor; and spring means for maintaining said balls in electricalcontact with said case and said second conductor when said pellet issolid and for breaking said electrical contact when said pellet melts.2. The thermal circuit protector as claimed in claim 1, wherein saidspring means comprises: a pressure spring exerting a first force on saidconductive balls; and a release spring exerting a second force on saidconductive balls, said second force opposing said first force, saidsecond force being smaller than said first force when said pellet issolid, said second force being larger than said first force after saidpellet melts to disengage said conductive balls and said secondconductor.
 3. The thermal circuit protector as claimed in claim 2,wherein an insulating member is disposed between said conductive ballsand said release spring.